Strain gauge with improved stability

ABSTRACT

A strain gauge includes a flexible substrate and a functional layer formed of a metal, an alloy, or a metal compound, directly on one surface of the substrate. The strain gauge includes a resistor formed of a film including Cr, CrN, and Cr 2 N, on one surface of the functional layer. The substrate has an expansion coefficient in a range of from 7 ppm/K to 20 ppm/K.

TECHNICAL FIELD

The present invention relates to a strain gauge.

BACKGROUND ART

A strain gauge is known to be attached to a measured object, to detectstrain on the measured object. The strain gauge includes a resistor fordetecting strain, and as a resistor material, for example, materialincluding Cr (chromium) or Ni (nickel) is used. The resistor is formedon a substrate made of, for example, an insulating resin (see, forexample, Patent document 1).

CITATION LIST Patent Document

[Patent document 1] Japanese Unexamined Patent Application PublicationNo. 2016-74934

SUMMARY

However, unlike a case of using a substrate formed of ceramics, etc.with high mechanical strength, when a flexible substrate is used, thereis a problem in warp formed in a strain gauge. If warp occurs in thestrain gauge, gauge characteristics may deteriorate due to cracksappearing in a resistor, or the strain gauge may not serve as a straingauge.

In view of the point described above, an object of the present inventionis to reduce the warp in a strain gauge including a resistor formed onor above a flexible substrate.

A strain gauge includes a flexible substrate; and a resistor formed ofmaterial including at least one from among chromium and nickel, on orabove the substrate, wherein the substrate has an expansion coefficientin a range of from 7 ppm/K to 20 ppm/K.

Effects of the Invention

According to the disclosed technique, with respect to a strain gaugeincluding a resistor formed on or above a flexible substrate, the warpcan be reduced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plan view of an example of a strain gauge according to afirst embodiment;

FIG. 2 is a cross-sectional view (part 1) of an example of the straingauge according to the first embodiment;

FIG. 3 is a cross-sectional view (part 2) of an example of the straingauge according to the first embodiment; and

FIG. 4 is a diagram illustrating a relationship between an expansioncoefficient of a substrate and internal stress of a resistor.

DESCRIPTION OF EMBODIMENTS

One or more embodiments will be hereinafter described with reference tothe drawings. In each figure, the same numerals denote the samecomponents; accordingly, duplicative explanations may be omitted.

First Embodiment

FIG. 1 is a plan view of an example of a strain gauge according to afirst embodiment. FIG. 2 is a cross-sectional view of an example of thestrain gauge according to the first embodiment, and illustrates a crosssection taken along the A-A line in FIG. 1. With reference to FIGS. 1and 2, the strain gauge 1 includes a substrate 10, a resistor 30, andterminal sections 41.

Note that in the present embodiment, for the sake of convenience, withrespect to the strain gauge 1, the side of the substrate 10 where theresistor 30 is provided is referred to as an upper side or one side; andthe side of the substrate 10 where the resistor 30 is not provided isreferred to as a lower side or another side. Further, for eachcomponent, the surface on the side where the resistor 30 is provided isreferred to as one surface or an upper surface; and the surface on theside where the resistor 30 is not provided is referred to as anothersurface or a lower surface. However, the strain gauge 1 can be used in astate of being upside down, or be disposed at any angle. Further, a planview means that an object is viewed from a normal direction of an uppersurface 10 a of the substrate 10, and a planar shape refers to a shapeof an object when viewed from the normal direction of the upper surface10 a of the substrate 10.

The substrate 10 is a member that is a base layer for forming theresistor 30 or the like and is flexible. The thickness of the substrate10 is not particularly restricted, and can be appropriately selected forany purpose. For example, such a thickness can be approximately between5 μm and 500 μm. In particular, when the thickness of the substrate 10is between 5 μm and 200 μm, it is preferable in terms of strain transferfrom a flexure element surface that is bonded to a lower surface of thesubstrate 10 via an adhesive layer or the like; and dimensionalstability with respect to environment, and when the thickness is 10 μmor more, it is further preferable in terms of insulation.

The substrate 10 can be formed of an insulating resin film such as a PI(polyimide) resin, an epoxy resin, a PEEK (polyether ether ketone)resin, a PEN (polyethylene naphthalate) resin, a PET (polyethyleneterephthalate) resin, a PPS (polyphenylene sulfide) resin, or apolyolefin resin. Note that the film refers to a flexible member havinga thickness of about 500 μm or less.

Here, the “formed of an insulating resin film” is not intended topreclude the substrate 10 from containing fillers, impurities, or thelike in the insulating resin film. The substrate 10 may be formed of,for example, an insulating resin film containing fillers such as silicaor alumina.

The resistor 30 is a thin film formed in a predetermined pattern and isa sensitive section where resistance varies according to strain. Theresistor 30 may be formed directly on the upper surface 10 a of thesubstrate 10, or be formed above the upper surface 10 a of the substrate10, via other layer (s). Note that in FIG. 1, for the sake ofconvenience, the resistor 30 is illustrated in a crepe pattern.

The resistor 30 can be formed of, for example, material including Cr(chromium); material including Ni (nickel); or material including bothof Cr and Ni. In other words, the resistor 30 can be formed of materialincluding at least one from among Cr and Ni. An example of the materialincluding Cr includes a Cr composite film. An example of the materialincluding nickel includes Cu—Ni (copper nickel). An example of thematerial including both of Cr and Ni includes Ni—Cr (nickel chromium).

Here, the Cr composite film is a composite film of Cr, CrN, Cr₂N, andthe like. The Cr composite film may include incidental impurities suchas chromium oxide.

The thickness of the resistor 30 is not particularly restricted, and canbe appropriately selected for any purpose. The thickness can be, forexample, approximately between 0.05 μm and 2 μm. In particular, when thethickness of the resistor 30 is 0.1 μm or more, it is preferable interms of improvement in crystallinity (e.g., crystallinity of α-Cr) of acrystal that constitutes the resistor 30, and when the thickness of theresistor 30 is 1 μm or less, it is further preferable in terms ofreduction in cracks of a given film caused by internal stress of thefilm that constitutes the resistor 30, or reduction in warp in thesubstrate 10.

For example, when the resistor 30 is the Cr composite film, the resistoris formed with α-Cr (alpha-chromium) as the main component having astable crystalline phase, so that stability of the gauge characteristicscan be improved. Additionally, when the resistor 30 is formed with α-Cras the main component, the gauge factor of the strain gauge 1 can be 10or more, as well as a gauge factor temperature coefficient TCS andtemperature coefficient of resistance TCR can be each in the range offrom −1000 ppm/° C. to +1000 ppm/° C. Here, a main component means thata target substance has 50% by weight or more of total substances thatconstitute the resistor. The resistor 30 preferably includes α-Cr of 80%by weight or more, from the viewpoint of improving the gaugecharacteristics. Mote that α-Cr is Cr having a bcc structure(body-centered cubic structure).

Note that the expansion coefficient of the substrate 10 is preferablybetween 7 ppm/K and 20 ppm/K, from the viewpoint of reducing warp in thesubstrate 10, where the internal stress of the resistor 30 is assumed tobe close to zero. The expansion coefficient of the substrate 10 can beadjusted by, for example, selecting the material of the substrate 10,selecting the material of the filler contained in the substrate 10,adjusting the content, and the like.

The terminal sections 41 respectively extend from both end portions ofthe resistor 30 and are each wider than the resistor 30 to be in anapproximately rectangular shape, in a plan view. The terminal sections41 are a pair of electrodes from which a change in a resistance value ofthe resistor 30 according to strain is output externally, where, forexample, a lead wire for an external connection, or the like is joined.For example, the resistor 30 extends zigzagged back and forth from oneof the terminal sections 41 to another terminal section 41. The uppersurface of each terminal section 41 may be coated with a metal allowingfor better solderability than the terminal section 41. Mote that for thesake of convenience, the resistor 30 and the terminal sections 41 areexpressed by different numerals. However, the resistor and the terminalsections can be integrally formed of the same material, in the sameprocess.

A cover layer 60 (insulating resin layer) may be disposed on and abovethe upper surface 10 a of the substrate 10, such that the resistor 30 iscoated and the terminal sections 41 are exposed. With the cover layer 60being provided, mechanical damage, and the like can be prevented fromoccurring in the resistor 30. Additionally, with the cover layer 60being provided, the resistor 30 can be protected against moisture, andthe like. Note that the cover layer 60 may be provided to cover allportions except for the terminal sections 41.

The cover layer 60 can be formed of an insulating resin such as a PIresin, an epoxy resin, a PEEK resin, a PEN resin, a PET resin, or a PPSresin, a composite resin (e.g., a silicone resin or a polyolefin resin).The cover layer 60 may contain fillers or pigments. The thickness of thecover layer 60 is not particularly restricted, and can be appropriatelyselected for any purpose. For example, the thickness may beapproximately between 2 μm and 30 μm.

In order to manufacture the strain gauge 1, first, the substrate 10 isprepared and the resistor 30 and the terminal sections 41 each of whichhas the planar shape illustrated in FIG. 1 are formed. The material andthickness for each of the resistor 30 and the terminal sections 41 arethe same as the material and thickness described above. The resistor 30and the terminal sections 41 can be integrally formed of the samematerial.

The resistor 30 and the terminal sections 41 are formed, for example,such that a raw material capable of forming the resistor 30 and theterminal sections 41 is the target to be deposited by magnetronsputtering, and such that patterning is performed by photolithography.Instead of the magnetron sputtering, the resistor 30 and the terminalsections 41 may be deposited by reactive sputtering, vapor deposition,arc ion plating, pulsed laser deposition, or the like.

From the viewpoint of stabilizing the gauge characteristics, beforedepositing the resistor 30 and the terminal sections 41, preferably, asa base layer, a functional layer having a film thickness that isapproximately between 1 nm and 100 nm is vacuum-deposited on the uppersurface 10 a of the substrate 10, by conventional sputtering, forexample. Note that, after forming the resistor 30 and the terminalsections 41 on the entire upper surface of the functional layer, thefunctional layer, as well as the resistor 30 and the terminal sections41, are patterned in the planar shape illustrated in FIG. 1, byphotolithography.

In the present application, the functional layer refers to a layer thathas a function of promoting crystal growth of the resistor 30 that is atleast an upper layer. The functional layer preferably further has afunction of preventing oxidation of the resistor 30 caused by oxygen andmoisture included in the substrate 10, as well as a function ofimproving adhesion between the substrate 10 and the resistor 30. Thefunctional layer may further have other functions.

The insulating resin film that constitutes the substrate 10 containsoxygen and moisture. In this regard, particularly when the resistor 30includes Cr, it is effective for the functional layer to have a functionof preventing oxidation of the resistor 30, because Cr forms anautoxidized film.

The material of the functional layer is not particularly restricted aslong as it is material having a function of promoting crystal growth ofthe resistor 30 that is at least an upper layer. Such material can beappropriately selected for any purpose, and includes one or more typesof metals selected from a group consisting of, for example, Cr(chromium), Ti (titanium), V (vanadium), Nb (niobium), Ta (tantalum), Ni(nickel), Y (yttrium), Zr (zirconium), Hf (hafnium), Si (silicon), C(carbon), Zn (zinc), Cu (copper), Bi (bismuth), Fe (iron), Mo(molybdenum), W (tungsten), Ru (ruthenium), Rh (rhodium), Re (rhenium),Os (osmium), Ir (iridium), Pt (platinum), Pd (palladium), Ag (silver),Au (gold), Co (cobalt), Mn (manganese), and Al (aluminum); an alloy ofany metals from among the group; or a compound of any metal from amongthe group.

Examples of the above alloy include FeCr, TiAl, FeNi, NiCr, CrCu, andthe like. Examples of the above compound include TiN, TaN, Si₃N₄, TiO₂,Ta₂O₅, SiO₂, and the like.

The functional layer can be vacuum-deposited by, for example,conventional sputtering in which a raw material capable of forming thefunctional layer is the target and in which an Ar (argon) gas issupplied to a chamber. By using conventional sputtering, the functionallayer is deposited while the upper surface 10 a of the substrate 10 isetched with Ar. Thus, a deposited amount of film of the functional layeris minimized and thus an effect of improving adhesion can be obtained.

However, this is an example of a method of depositing the functionallayer, and the functional layer may be formed by other methods. Forexample, before depositing the functional layer, the upper; surface 10 aof the substrate 10 is activated by plasma treatment, etc. using Ar, orthe like to thereby obtain the effect of improving the adhesion;subsequently, the functional layer may be vacuum-deposited by magnetronsputtering.

A combination of the material of the functional layer and the materialof the resistor 30 and the terminal sections 41 is not particularlyrestricted, and can be appropriately selected for any purpose. Forexample, Ti is used for the functional layer, and a Cr composite filmformed with α-Cr (alpha-chromium) as the main component can be depositedas the resistor 30 and the terminal sections 41.

In this case, each of the resistor 30 and the terminal sections 41 canbe deposited by, for example, magnetron sputtering in which a rawmaterial capable of forming the Cr composite film is the target and inwhich an Ar gas is supplied to a chamber.

Alternatively, the resistor 30 and the terminal sections 41 may bedeposited by reactive sputtering in which pure Cr is the target and inwhich an appropriate amount of nitrogen gas, as well as an Ar gas, aresupplied to a chamber.

In such methods, a growth face of the Cr composite film is defined bythe functional layer formed of Ti, and a Cr composite film that isformed with α-Cr as the main component having a stable crystallinestructure can be deposited. Also, Ti that constitutes the functionallayer is diffused into the Cr composite film, so that the gaugecharacteristics are improved. For example, the gauge factor of thestrain gauge 1 can be 10 or more, as well as the gauge factortemperature coefficient TCS and temperature coefficient of resistanceTCR can be each in the range of from −1000 ppm/° C. to +1000 ppm/° C.Note that, when the functional layer is formed of Ti, the Cr compositefilm may include Ti or TiN (titanium nitride).

Note that when the resistor 30 is a Cr composite film, the functionallayer formed of Ti includes all functions being a function of promotingcrystal growth of the resistor 30; a function of preventing oxidation ofthe resistor 30 caused by oxygen or moisture contained in the substrate10; and a function of improving adhesion between the substrate 10 andthe resistor 30. Instead of Ti, when the functional layer is formed ofTa, Si, Al, or Fe, the functional layer also includes the samefunctions.

As described above, with the functional layer being provided in thelower layer of the resistor 30, the crystal growth of the resistor 30can be promoted and thus the resistor 30 having a stable crystallinephase can be fabricated. As a result, with respect to the strain gauge1, the stability of the gauge characteristics can be improved. Also, thematerial that constitutes the functional layer is diffused into theresistor 30, so that the gauge characteristics of the strain gauge 1 canbe thereby improved.

After forming the resistor 30 and the terminal sections 41, the coverlayer 60 with which the resistor 30 is coated and that exposes theterminal sections 41 is formed on and above the upper surface 10 a ofthe substrate 10, as necessary, so that the strain gauge 1 is completed.For example, the cover layer 60 can be fabricated, such that athermosetting insulating resin film in a semi-cured state is laminatedon the upper surface 10 a of the substrate 10, and such that theresistor 30 is coated therewith and the terminal sections 41 areexposed; subsequently, heat is added and curing is performed. The coverlayer 60 may be formed, such that a thermosetting insulating resin thatis liquid or paste-like is applied to the upper surface 10 a of thesubstrate 10, and such that the resistor 30 is coated therewith and theterminal sections 41 are exposed; subsequently, heat is added and curingis performed.

Note that when the functional layer, as a base layer of the resistor 30and the terminal sections 41 is provided on the upper surface 10 a ofthe substrate 10, the strain gauge 1 has a cross-section shapeillustrated in FIG. 3. A layer expressed by the numeral 20 indicates thefunctional layer. The planar shape of the strain gauge 1 in the case ofproviding the functional layer 20 is the same as that in FIG. 1.

Example 1

In Example 1, multiple substrates 10 each formed of a polyimide resinthat had a thickness of 25 μm and that had a different expansioncoefficient were prepared. Then, when a Cr-composite film, as a givenresistor 30, was deposited, a relationship between an expansioncoefficient of a given substrate 10 and internal stress of the resistor30 was checked, to thereby obtain the result illustrated in FIG. 4.

The internal stress of the resistor 30 was estimated by measuring warpin an evaluation sample and using the Stoney formula given by Formula(1). Note that as can be seen from Formula (1), the internal stress ofthe resistor 30 illustrated in FIG. 14 indicates a value per unitthickness and does not depend on the thickness of the resistor 30.[Math. 1]INTERNAL STRESS=ED2/6(1−v)tR  (1)Note that in Formula (1), E denotes Young's modulus, v denotes Poisson'sratio, D denotes the thickness of the substrate 10, t denotes thethickness of the resistor 30, and R denotes change in radius ofcurvature in the substrate 10.

From FIG. 4, when the expansion coefficient of the substrate 10 is inthe range of from 7 ppm/K to 20 ppm/K, the internal stress of theresistor 30 can be maintained to be in the range of ±0.4 GPa. Where,±0.4 GPa indicates values expressing a permittable warp in the straingauge 1 for functioning, and was experimentally determined by theinventors.

In other words, when the expansion coefficient of the substrate 10 isout of the range of from 7 ppm/K to 20 ppm/K, the internal stress of theresistor 30 is out of the range of ±0.4 GPa and thus warp in the straingauge 1 would increase, so that the strain gauge 1 would not function asa strain gauge. Therefore, the expansion coefficient of the substrate 10is required to be in the range of from 7 ppm/K to 20 ppm/K. Note thatthe material of the substrate 10 does not necessarily include apolyimide resin.

The expansion coefficient of the substrate 10 can be in the range offrom 7 ppm/K to 20 ppm/K, by selecting the material of the substrate 10,selecting the material of the filler contained in the substrate 10,adjusting the content, and the like.

As described above, with the expansion coefficient of the substrate 10being in the range of from 7 ppm/K to 20 ppm/K, a difference in theexpansion coefficient between the substrate 10 and the resistor 30, aswell as other factors, are absorbed, so that the internal stress of theresistor 30 can be in the range of ±0.4 GPa. As a result, warp in thestrain gauge 1 is reduced to thereby cause the strain gauge 1 to be ableto function stably in a manner such that good gauge characteristics aremaintained.

Example 2

In Example 2, multiple strain gauges 1 were each fabricated using agiven substrate 10 that had an expansion coefficient in the range offrom 7 ppm/K to 20 ppm/K.

First, Ti as the functional layer 20, which had a film thickness of 3nm, was vacuum-deposited on the upper surface 10 a of the substrate 10that was formed of a polyimide resin having a thickness of 25 μm, byconventional sputtering.

Subsequently, a Cr composite film, as the resistor 30 and the terminalsections 41, was deposited on the entire upper surface of the functionallayer 20, by magnetron sputtering, and then the functional layer 20, theresistor 30, and the terminal sections 41 were patterned byphotolithography, as illustrated in FIG. 1.

Then, the gauge characteristics of each sample in Example 2 weremeasured. As a result, the gauge factor of each sample in Example 2 wasbetween 14 and 16. Also, for each sample in Example 2, the gauge factortemperature coefficient TCS and temperature coefficient of resistanceTCR were each in the range of from −1000 ppm/° C. to +1000 ppm/° C.

As described above, when the substrate 10 having an expansioncoefficient, in the range of from 7 ppm/K to 20 ppm/K was used, it wasconfirmed that warp was reduced so that the strain gauge 1 having goodgauge characteristics could be fabricated. Further, it was confirmedthat the presence of the functional layer 20 did not result indeterioration of warp in a given strain gauge 1.

The preferred embodiment, and the like have been described above indetail, but are not limited thereto. Various modifications andalternatives to the above embodiment and the like can be made withoutdeparting from a scope set forth in the claims.

This International application claims priority to Japanese PatentApplication No. 2017-191821, filed Sep. 29, 2017, the contents of whichare incorporated herein by reference in their entirety.

REFERENCE SIGNS LIST

1 strain gauge, 10 substrate, 10 a upper surface, 20 functional layer,30 resistor, 41 terminal section, 60 cover layer

The invention claimed is:
 1. A strain gauge comprising: a flexible resinsubstrate; a functional layer formed of a metal, an alloy, or a metalcompound, directly on one surface of the substrate; and a resistorformed of a film that includes Cr, CrN, and Cr₂N and into which anelement included in the functional layer is diffused, on one surface ofthe functional layer, wherein a gauge factor of the strain gauge is 10or more, and wherein the substrate has an expansion coefficient of thestrain gauge in a range of from 7 ppm/K to 20 ppm/K.
 2. The strain gaugeaccording to claim 1, wherein the functional layer includes one or moremetals selected from the group consisting of Cr, Ti, V, Nb, Ta, Ni, Y,Zr, Hf, Si, C, Zn, Cu, Bi, Fe, Mo, W, Ru, Rh, Re, Os, Ir, Pt, Pd, Ag,Au, Co, Mn, and Al; an alloy of any metals from among the group; or acompound of any metal from among the group.
 3. The strain gaugeaccording to claim 2, wherein the functional layer includes one or moremetals selected from the group consisting of Cr, V, Nb, Ta, Ni, Y, Hf,C, Zn, Bi, Fe, Mo, W, Ru, Rh, Re, Os, Ir, Pt, Pd, Ag, Au, Co, and Mn; analloy of any metals from among the group; or a compound of any metalfrom among the group.
 4. The strain gauge according to claim 2, whereinthe functional layer includes one metal compound selected from the groupconsisting of TiN, TaN, Si₃N₄, TiO₂, Ta₂O₅, and SiO₂.
 5. The straingauge according to claim 4, wherein the functional layer includes onemetal compound selected from the group consisting of TiN, TaN, Si₃N₄,and Ta₂O₅.
 6. The strain gauge according to claim 2, wherein thefunctional layer includes one alloy selected from the group consistingof FeCr, TiAl, FeNi, NiCr, and CrCu.
 7. The strain gauge according toclaim 1, wherein the functional layer protects the resistor fromoxidation; suppresses movement of oxygen and moisture present in thesubstrate into the resistor; and/or improves adhesion between thesubstrate and the resistor.
 8. The strain gauge according to claim 1,wherein the functional layer is patterned in a same planar shape as theresistor.
 9. The strain gauge according to claim 1, wherein thefunctional layer has a thickness of from 1 nm to 100 nm.
 10. A straingauge comprising: a flexible resin substrate; a functional layer formedof a metal, an alloy, or a metal compound, directly on one surface ofthe substrate; and a resistor formed of a film that includes Cr, CrN,and Cr₂N and into which an element included in the functional layer isdiffused, on one surface of the functional layer, wherein a temperaturecoefficient of resistance of the strain gauge is in a range of from−1000 ppm/° C. to +1000 ppm/° C., and wherein the substrate has anexpansion coefficient in a range of from 7 ppm/K to 20 ppm/K.
 11. Astrain gauge comprising: a flexible resin substrate; a functional layerformed of a metal, an alloy, or a metal compound, directly on onesurface of the substrate; and a resistor formed of a film including Cr,CrN, and Cr₂N, on one surface of the functional layer, wherein thesubstrate has an expansion coefficient in a range of from 7 ppm/K to 20ppm/K.